RankingAgorithm is a search library that uses a new scoring algorithm to rank results accurately and relevantly. RankingAlgorithm is very easy to use since it uses the Apache Lucene** index but ranks and scores on its own.

Multiple algorithms are available, SIMPLE* and COMPLEX*. SIMPLE is a very fast algorithm and can return queries in <10ms on a 10m wikipedia index (complete index).
It can also scale to 2 billion docs. COMPLEX is a more complex algorithm so is a little
slower compared to the SIMPLE, but can also still return queries in < 10ms on a 10m wikipedia index (complete index).
COMPLEX is more accurate and should be able to give you the best rankings as compared to SIMPLE. SIMPLE is also very good and may be well suited than COMPLEX for some type of queries.

In document mode, it ranks documents such as HTML, Wikipedia, Word/PDF docs relevantly while in Product mode, a term's occurrence is taken into account and scored accordingly. So titles starting with "wii console" are ranked first, and the others rank lower as the occurrence of "wii console" shifts in the title or gets reversed, see below:

Wii Console and Wii Fit Plus with Balance Board Bundle (Nintendo Wii)

Wii Console System with Wii Sports Resort Game with TWO MotionPlus Attachments

RankingAlgorithm is very easy to use as Apache Lucene the very popular open source library can be used create the index. The results from Solr (again a very popular open source search platform) with RankingAlgorithm seems to be comparable to Google and much better than Lucene for Perl index searches, see comparison.

Autocomplete using Solr with RankingAlgorithm, similar to Google/Yahoo/Bing's autocomplete (It is free), Give it a try
Autocomplete components bundled with RankingAlgorithm (very fast), similar to Google/Yahoo/Bing's autocomplete (It is free), Give it a try

ver 1.4.8, works with Lucene 4.3 [md5sum:a40e448358657de69291ee1c517d3a64], 2013/06/02 (1.4.8.20130602.1)
(Has built in components Edge/Infix autocomplete with source. Support for glob/regular expressions in boolean queries)

ver 1.4.7, works with Lucene 4.1/4.0 [md5sum:ea6d5ca2c727026a05778827], 2013/03/18 (1.4.7.20130318.1)
(H/as built in components Edge/Infix autocomplete with source. Support for glob/regular expressions in boolean queries)

ver 1.4.6, works with Lucene 4.x [md5sum:61cab6301d30fa681ab198cf57355e42], 2012/12/23 (1.4.5.20121105.6)
(Has built in components Edge/Infix autocomplete with source. Support for glob/regular expressions in boolean queries)

ver 1.4.5, works with Lucene 4.x [md5sum:97ff962f5bb1a060591f35e07effa8fd], 2012/12/09 (1.4.5.20121105.4)
(Has built in components Edge/Infix autocomplete with source. Support for glob/regular expressions in boolean queries)